Distributions of Relative Humidity, Vertical Velocity, and Chemical Tracers in the Tropical Tropopause Layer From ATTREX and CONTRAST Campaigns and Their Representation in Numerical Models

Ice clouds remain an uncertainty for Earth’s changing climate due to the two different roles they play in Earth’s energy balance. These clouds have the ability to reflect incoming solar radiation and trap outgoing longwave radiation from Earth’s surface, depending on the cloud microphysical properties. This causes the net radiative effects from ice clouds to be largely variable. A key step to estimate how ice clouds will affect Earth’s climate is to improve the understanding of ice cloud formation.

Recent research flight campaigns in the region of the tropical western Pacific, in order to collect in-situ observational data over a remote area of warm ocean waters . Using the in-situ data from two flight campaigns, the NASA Airborne Tropical Tropopause Experiment (ATTREX) and the NSF Convective Transport of Active Species in the Tropics (CONTRAST), analysis of ice cloud formation in the tropical tropopause layer will be performed. WRF-Chem will be used to identify if the microphysical properties observed in the in-situ data are represented in the model data.

Previous analysis performed by Diao et al. (2015) showed that ice supersaturation (ISS) most frequently occurs around and below the extratropical tropopause layer.  Similarly, our analysis of the ATTREX data shows that the highest frequency of ISS occurs around the lowest temperatures (around -90°C to  -88°C), while a moderate amount of ISS occurs between -88°C and -70°C, and the lowest amount of ISS occurs between -70°C and -60°C. The results show that the convective activities in the TTL could be a main factor controlling the coexistence of ISS and ice clouds. In addition, our analysis of the CONTRAST data shows that at temperatures below -40°C, relatively lower average vertical velocity (~ -0.3 m/s) and lower frequency of updrafts (~ 30%) were observed at RH with respect to ice (RH_i) < 100%, compared to those where RH_i > 100% (~ 0.25 m/s and ~ 70%, respectively). These results indicate correlations between RH_iand updrafts in the TTL. We further analyze the correlation between ISS / total ISS occurrences are associated with higher carbon monoxide concentration (60 to 90 ppbv) and higher average vertical velocities (about 0.6 m/s to 1.2 m/s). On the other hand, clear-sky ISS / total ISS occurrences were found to be associated with lower carbon monoxide concentrations (< 60 ppbv) and relatively lower vertical velocities of +/-0.2m/s.

When analyzing the correlation between RH_i and chemical tracers, we found that an increase in ozone concentration is correlated with a decrease in RH values in clear-sky conditions for certain temperature ranges (> 0°C and 0°C to -40°C), but such correlation is not clearly observed at temperatures below -40°C. In addition, regions with high RH_i values (≥ 20%) were found to have lower potential temperatures than those of low RH_i values (< 20%), which are ~ 353 K and ~ 360 K, respectively. Finally, WRF-Chem will be used compare the results from the simulation to ozone and carbon monoxide concentrations and RH_i and vertical velocity from the in-situ observations, in order to evaluate the correlations among ISS, vertical velocity, and chemical tracers in numerical models.